In an era of cookie-cutter movies, each tailored to a one-word genre, it’s actually a little stirring to encounter a project so deeply mired in an identity crisis.

Ben Stiller’s latest directorial exercise looks a little like a romantic comedy: the comic plays a lonely drone much in love with new colleague Kristen Wiig. The film has something of the sentimental “you can do it” drama about it. At times it turns positively arty. You certainly couldn’t say that it was cynically honed for a target demographic.

Among the few things the film can’t claim to be is any meaningful version of James Thurber’s The Secret Life of Walter Mitty (or the 1947 Danny Kaye film it inspired). That brief tale followed a hen-pecked man as, seeking escape from humdrum reality, he imagined himself a hydroplane captain, a top surgeon and the witness in a murder case. There is nothing to the story but that contrast between the romantic and the mundane.

Long in development, Stiller’s film has imposed an unusual tale concerning the last days of Life magazine (which actually closed in 2000) onto that fragile foundation. An employee in the photo department, Walter is obsessed with tracking down a missing, apparently “quintessential” image taken by the organ’s most glamorous, elusive photojournalist (Sean Penn in rugged mode). After evading corporate takeover drones – who, hilariously, now all wear beards with suits – Walter elects to seek out the snapper in Greenland.

By this stage, the new story has pushed Thurber rudely into the gutter and completely taken over the action. The initial crop of elaborate fantasies – including a truly weird, deeply creepy riff on Benjamin Button – fades away as Walter goes on to live a life more exciting than 90 per cent of the film’s viewers. That clip you’ve seen of him evading sharks is not a fantasy.

So it abandons its source material? Big deal. Beautifully made and featuring nicely dry performances by Stiller and Wiig, The Secret Life of Walter Mitty probably succeeds on its own terms. Given the elusive nature of those terms, it is, however, hard to tell for certain.